WO2000003041A1

WO2000003041A1 - Flat product, such as sheet metal, made of steel with high yield strength having good ductility and method for making same

Info

Publication number: WO2000003041A1
Application number: PCT/BE1999/000088
Authority: WO
Inventors: Klaus Eberle; Philippe Harlet; Pierre Cantinieaux
Original assignee: Recherche Et Developpement Du Groupe Cockerill Sambre, Rd-Cs
Priority date: 1998-07-08
Filing date: 1999-07-07
Publication date: 2000-01-20
Also published as: AU4764999A; CA2337260A1

Abstract

The invention concerns a flat product made of multiphase steel and a method for preparing said steel comprising the following steps: hot rolling at a temperature at which the austenitic phase is stable; tempering to form a C and Mn-enriched phase in a ferrite matrix; heat treatment to form austenite islands and/or enriching in Mn the already formed austenite and cooling down to room temperature so as to obtain a final product with a ferrite matrix containing residual austenite, bainite and/or martensite islands.

Description

"Flat product, such as sheet metal, of a high yield strength steel showing good ductility and method of manufacturing this product."

The present invention relates to a flat product, such as a steel sheet, containing Mn, with multiple phases with a ferrite matrix.

High-strength steels such as rephosphorus steels, microalloyed steels, "bake-hardening" steels, are widely used for automobile parts. Sheets made of such steel require sufficient strength to meet the safety of automobiles and must, moreover, have excellent shaping properties.

It is also known that the strength and ductility of a multiphase steel can be improved by a multiphase microstructure possibly combined with a transformation induced plasticity ("transformation induced plasticity," TRIP ") from residual austenite to martensite.

The "TRIP" effect was first discovered by Zackay et al in steels containing large amounts of nickel and chromium.

However, the presence in these steels of large quantities of such alloying elements poses problems for the manufacture of steels under economically profitable conditions.

It should also be noted that a steel containing a significant amount of residual austenite can be obtained by the addition of silicon and manganese and by hot rolling with a controlled thermal cycle or by cold rolling followed by an intercritical annealing combined with a bainitic holding producing a structure formed of several phases and with isolated zones or islands of residual austenite which is stable at room temperature.

Such steel is known in particular from document US-A-4,544,422 and the articles by Gregory H. Haidemonopoulos "Austenite stabilization from direct cementite conversation in low-alloy steels", in Steel Research 67 (1996) n ° 3, p . 93 to 99, and "Modeling of austenite stability in low-alloy triple-phase steels" Steel Research 67 (1996) n ° 11, p. 513 to 519.

These known steels and their production, however, have certain significant drawbacks. In fact, these steels generally require large quantities of addition elements. In addition, the fact that in these known steels the silicon content is generally relatively high, of the order of 1.25 to 1.50% of Si, gives a steel sheet having certain surface defects called "languages of cat ", which are created during hot rolling. Furthermore, these known steels pose problems during metal dipping, such as galvanizing, due to the embrittlement of the coating and to the problems of wettability of the liquid metal intended to form the coating.

One of the essential aims of the present invention is to provide a sheet of steel which makes it possible to remedy the abovementioned drawbacks and which, therefore, is particularly suitable for galvanizing while being able to be manufactured according to an economically very favorable process.

The steel according to the invention has improved properties of strength and ductility, while having an extremely reduced silicon content, so that it is suitable particularly for shaping and surface treatment in the automotive industry.

To this end, the steel sheet according to the invention has a structure and properties which can be obtained by a process as defined in claim 1.

Advantageously, the steel according to the invention has the following chemical composition: 0.05 to 0.8% of C; 0.2 to 3.0% Mn;

If <1.0%; B ≤ 0.100%; Ti, Nb, Zr and V each <0.200%, Al <0.400%;

N <0.100%; P <0.100%; Cr, Ni and Cu each <2,000%; Mo <0.500%.

The invention also relates to the aforementioned particular process for the manufacture of the flat steel product, according to the invention.

According to a particular embodiment of this process, the abovementioned tempering is carried out at a temperature below the eutectoid temperature (Ai), so as to form cementite and to cause the diffusion of carburogenic elements, such as Mn in this cementite.

According to a preferred embodiment, the method according to the invention comprises a step of cooling to room temperature after the above-mentioned holding or tempering step, this step then being followed by a cold rolling step before the 'step of the aforementioned heat treatment.

Other details and particularities of the invention will emerge from the description given below, by way of nonlimiting example, of some particular embodiments of the invention with reference to the appended drawings.

Figures 1 to 15 relate to schematic graphs of the treatment temperature in ° C, as a function of the steel treatment time, illustrating the different stages of the process according to the invention. Figures 16 to 23 are schematic representations of a cross section of different microstructures of the steel according to the invention.

In the various figures, the same reference numerals designate analogous or identical elements.

In general, the invention relates to a multi-phase steel preferably in the form of a sheet and comprising a ferrite matrix in which are distributed islands of at least one of the following phases: bainite, martensite or residual austenite which is also stable at room temperature, and which possibly shows an induced transformation of plasticity ("TRIP"). In addition, the ferrite matrix can be reinforced by secondary precipitates of the micro-alloy elements.

Chemically, this steel can contain from 0.05 to 0.8% carbon, 0.2 to 3.0% manganese and less than 1% silicon. However, according to the invention, preference is given to steels containing as little silicon as possible, for example less than

0.5%, preferably at most 0.4% or even less than 0.2%.

In addition to the above elements, this steel may have a deliberately adjusted low content of boron, titanium, niobium, zirconium, vanadium, aluminum, nitrogen, phosphorus, chromium, nickel, copper, molybdenum and traces of impurities generally unavoidable during the steel preparation, the rest being iron.

More particularly, for the steel according to the invention, the concentration of the various aforementioned elements is advantageously as follows: B <0.100%; Ti, Nb, Zr and V each <0.200%, Al <0.400%; N <0.100%; P <0.100%; Cr, Ni and Cu <2,000% each; Mo <0.500%.

As already mentioned above, the invention also relates to a process for the manufacture of a multi-phase steel sheet corresponding to the aforementioned chemical composition, according to which one subjects this steel for example in the form of a slab, in the following successive stages:

the hot rolling of this steel at a temperature at which the austenitic phase is stable, that is to say at a temperature above the transformation temperature of austenite (A3),

- tempering or maintaining the wound sheet at a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (A ^ for at least 4 hours, possibly followed by cooling to the ambient temperature, so as to form a phase enriched with carburogenic and / or gamma-genic elements, such as C and Mn in a ferrite matrix,

a heat treatment at a temperature higher than the above eutectoid temperature (Ai) and lower than the austenite formation temperature (A ₃ ) so as to form islands of austenite and / or to enrich with gamma elements, such that of the Mn of the austenite already formed, and

- subsequent cooling to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite.

The above heat treatment can be preceded or followed by cold rolling.

In fact, the invention consists in presenting a steel containing simultaneously at least two phases such as ferrite / residual austenite or ferrite / bainite, ferrite / martensite, ferrite / bainite / martensite with or without residual austenite and a very reduced silicon content, having, in addition, a structure and properties of a steel obtained by the process as defined above.

In this process, one can generally detect three important successive steps. In the first stage, a hot rolling of a steel takes place, corresponding to the aforementioned chemical composition, above the temperature A3, which is the temperature of transformation of austenite into ferrite and which depends on the chemical composition of steel. The heating of the bram takes place at temperatures from 1100 ° C to 1350 ° C. for 130 to 250 minutes. This step can be considered as the austenization step during which the formation of ferrite and perlite is avoided and where C and Mn are kept in solution. Hot rolling includes a roughing rolling followed by a finishing rolling which take place at a temperature higher than the temperature A3.

This hot rolling can then optionally be followed by cooling to room temperature, and this in such a way as to form a microstructure of the bainite and / or martensite, while avoiding the formation of ferrite and perlite. .

This cooling preferably takes place at least partially on a cooling table called "Runout table" or by an accelerated cooling tool called "Ultra Fast Cooling".

In the second step, after the winding of the sheet obtained by the aforementioned rolling, during which the sheet continues to cool, and after pickling, the tempering takes place which can, for example, be achieved by heating in an oven bell annealing up to a temperature between 300 ° C and 50 ° C above the eutectoid temperature A _L This temperature is maintained for at least 4 hours and preferably up to at most 200 hours.

For some thermomechanical paths developed, there is a combination of hot rolling directly followed by holding or tempering without cooling the sheet to room temperature. The flat product, during maintenance or tempering, is still covered on the surface by an iron oxide called "calamine". The advantages of calamine encountered during maintenance or tempering are as follows: no problem of bonding of the turns during maintenance or tempering, and no problem of decarburization of the surface of the sheet even without the use of a reducing atmosphere in the bell insulation or in the annealing bell.

In the third step, heat treatment takes place at a temperature higher than the eutectoid temperature Ai and lower than the austenite formation temperature A ₃ .

This heat treatment can be carried out either directly after tempering, or after cooling possibly combined with cold rolling. It is in any case followed by controlled cooling down to room temperature. The main purpose of this operation is to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite. The relative quantity of these phases is essentially a function of the starting chemical composition of the treated steel and of the particular conditions under which the different steps are carried out.

More particularly, the C content determines the maximum volume of ferrite and makes it possible to significantly decrease the starting martensite temperature Ms according to the formula:

M _s = 539 - 423 x C - 30.4 x Mn -7.5 x Si

(Andrews formula)

The C content controls the start time in a continuous cooling diagram and the kinetics of ferrite and bainite formation.

Thus, the choice of the C content makes it possible to obtain a microstructure of martensite and / or bainite during cooling on a wide band train table called "runout table" or by an accelerated cooling device called "Ultra Fast Cooling ". Carbon is an interstitial element which hardens the ferritic phases in steel. In an income microstructure, a carbon-rich phase called "iron carbide" or "cementite" is formed. By the carbon content, the fraction of this phase formed can be controlled. Manganese is known as an element which increases the hardenability of a steel and as a substitute element which hardens the ferritic phases in steel. Manganese can be enriched by exchange reaction in iron carbides (substitution of iron by manganese) during a tempering treatment. The choice of the manganese content makes it possible to influence the bainite formation zone in a continuous cooling diagram and in the isothermal maintenance diagram. Manganese makes it possible to reduce the starting temperature of the formation of martensite and therefore to improve the stability of the austenitic phase.

Silicon is a substitute element which hardens the ferritic phases in steel. It is alpha-gene, which means that it increases the temperature of ferrite formation during cooling. At a higher Si content, certain surface defects called "cat's tongue", which are created during hot rolling, may appear. Furthermore, these known steels pose problems during metal dipping, such as galvanizing, due to the embrittlement of the coating and to the problems of wettability of the liquid metal intended to form the coating. The silicon stabilizes the carbon in solution in the austenite and in the bainite by an inhibition of the precipitation of cementite. This effect is explained by the fact that silicon is relatively poorly soluble in cementite, which requires the controlled ejection, by diffusion, of silicon in the transformation front. This causes growth inhibition of the cementite embryos. The elements, consisting of niobium, vanadium, zirconium and titanium, alone or in combination, are used in small quantities to form carbides, nitrides or carbonitrides so as to be able to block the growth of grains during the heating of the slab and to be able to increase the resistance by the precipitation effect

During hot rolling, during cooling on the cooling table and / or in the coil after winding, an effect of increasing the resistance by the precipitation effect can take place. In the non-precipitated state, that is to say in the state in solution in the crystalline ferrite mesh, the microalloy elements can lead to an effect of increasing the resistance during a tempering treatment by secondary precipitation.

Aluminum is used to fix nitrogen in solution by forming aluminum nitrides. Aluminum nitrides also have a positive effect by reducing the increase in austenite grains up to a temperature of the order of 1150 ° C. during the heating of the steel during hot rolling.

Aluminum is a substitution element which hardens the ferritic phases in steel and which stabilizes the carbon in solution in austenite and bainite by inhibiting the precipitation of cementite. This effect is explained by the fact that aluminum is relatively poorly soluble in cementite, which requires controlled ejection, by diffusion, of aluminum in the transformation front. This causes growth inhibition of the cementite embryos. Phosphorus has a positive effect on the strength of steel, but must also be kept low enough to avoid embrittlement effects. Nitrogen is an impurity, the content of which must be kept as low as possible. Nitrogen is an interstitial element which hardens the ferritic phases in steel and, in solution in steel, it increases the sensitivity to aging.

In very small quantities, the boron is deposited in the grain boundaries and thus increases the ductility. Boron also affects the formation of phases, which are obtained by diffusion, such as the ferrite and perlite phases. Boron increases the hardenability of steel during hot rolling. Boron forms precipitates with nitrogen in solution. At a temperature at which bainite formation takes place, boron decreases the kinetics of bainite formation. Boron can be enriched by exchange reaction in iron carbides (substitution of carbon by boron) or by coupled precipitation of iron borocarbons and iron carbides during a tempering treatment at a sufficient activation temperature. .

Molybdenum is a substitute element, it hardens the ferritic phases in steel. Molybdenum is known as an element which increases the hardenability of a steel. The molybdenum can be enriched by exchange reaction in iron carbides (substitution of iron by molybdenum) or by coupled precipitation of iron carbides with molybdenum carbides during a tempering treatment at a sufficient activation temperature. .

Chromium is a substitute element which hardens the ferritic phases in steel. Chromium is known as an element which increases the hardenability of a steel and can be enriched by exchange reaction in iron carbides (substitution of iron by chromium) or by coupled precipitation of iron carbides with chromium carbides during a tempering treatment at a sufficient activation temperature. Nickel and copper are substitution elements which harden the ferritic phases in steel and which are known as elements which increase the hardenability of a steel.

It should also be noted that a heat treatment, such as tempering or annealing, at a temperature between temperatures Ai and A3 is called "intercritical tempering or annealing".

In some of the steps of the process described above as well as in some of the particular embodiments of this process described below, mention is made of so-called "slow" cooling, of so-called "rapid" cooling and / or so-called "accelerated" cooling.

Generally speaking, "slow cooling" generally corresponds to a decrease in temperature from 10 ° to 40 ° C per hour, depending on the thickness of the sheet, the composition of the steel and the initial temperature. sheet metal. On the other hand, by "rapid cooling", which generally takes place by means of a fluid, generally corresponds to a decrease in temperature from 10 to 300 ° C. per second, depending on the thickness of the sheet, the composition of the sheet and the initial temperature of the sheet. Accelerated cooling by quenching the coil wound in a liquid generally corresponds to a decrease in temperature with a speed greater than 10 ° C per minute.

The graphs represented diagrammatically in FIGS. 1 to 15 illustrate some particular embodiments of this process, which is applied to a slab or an ingot of steel corresponding to the above-mentioned composition and having followed a reheating at temperatures from 1100 ° C. to 1350 ° C for 130 to 250 minutes.

In the first embodiment according to FIG. 1, after the first step, comprising hot rolling in the austenitic region above the temperature A3, represented by the line segment 1, followed by cooling, represented by the segment right 2, to obtain a quenched structure of bainite and / or martensite, and the second step formed by tempering, represented by the reference 3, which takes place by annealing "bell" at a temperature between 300 ° C and the temperature A ^ for at least 4 hours, a sheet is obtained comprising iron carbides (cementite) in which the carburogenic elements, such as Mn has diffused, more particularly cementite enriched with manganese, chromium, molybdenum and / or boron and possibly secondary precipitates formed from micro-alloy of these elements in the ferrite matrix. By "bell" annealing, there is in fact meant a discontinuous annealing carried out on a sheet rolled up in a coil in an oven maintained at the desired temperature.

After cooling 5, slow or accelerated by quenching, the sheet was subjected to the third step, which comprises the aforementioned heat treatment 4, formed either by continuous annealing in the temperature zone situated between the temperature A ^ and A ₃ , preferably at most 900 ° C, for 1 second to 5 minutes, or annealing "bell" intercritical in the above temperature zone. During this annealing, the carbides (cementite) are transformed into austenite, the chemical composition of which will be enriched by the elements contained and the carbides with germination of the austenite at the joints of the cementite / ferrite grain. This annealing is then followed by rapid cooling, linked to the conventional technology of continuous annealing which is an annealing of an unrolled sheet, or slow or accelerated cooling by quenching, which, in this case, can be carried out. on a coil sheet.

In the case of continuous annealing, strips of unwound sheet metal are welded end to end so as to obtain in practice an endless strip. Thus, in such a case, the annealing time is extremely limited and is generally between a few seconds and a few minutes, while the annealing bell takes usually several hours. In the latter case, if necessary, accelerated quench cooling can also be applied.

Furthermore, if necessary, a pickling of the scale can be applied before or after the aforementioned second step. Pickling can be carried out either by a chemical attack on the scale in an acid bath or by a reduction of the scale during annealing in a reducing atmosphere rich in hydrogen.

Finally, the sheet is either uncoated, or coated by electrochemical metal deposition, or galvanized, for example after shaping.

FIG. 2 relates to a second embodiment of the method according to the invention which differs from that of FIG. 1 by the fact that the annealing 3 by bell annealing is carried out at a temperature below the temperature A1 and is immediately followed by the heat treatment 4 formed by an intercritical tempering by bell annealing between the temperature A and a temperature of 50 ° C. above this temperature Ai.

In this case, relatively slow heating can take place between tempering 3 and heat treatment 4, during which residual austenite is also formed. This heat treatment 4 is then followed by cooling 5, which can be slow cooling generally linked to conventional bell annealing technology or accelerated cooling by quenching.

The third embodiment, illustrated in FIG. 3, differs from the previous two in that the tempering 3, like the aforementioned heat treatment 4, takes place at the same temperature which is higher than the temperature A and lower than temperature A _T + 50 ° C, so as to constitute a single operation. During the heating, which precedes this operation, begins the formation of a microstructure of fine particles of cementite with cores for austenite germs during austenization at tempering temperature 3 and heat treatment

4. As in the previous embodiment, this operation is then followed by slow or accelerated cooling 5, as in the two previous embodiments.

The fourth embodiment, illustrated in FIG. 4, differs from the third embodiment in that the hot rolling 1 is first followed by cooling 2 and by winding at a temperature t ₂ , below the temperature at the start of the bainite formation, and then directly from the income 3, in particular from a "bell" annealing of the black coil, that is to say not pickled. Pickling can take place after cooling 5.

The fifth embodiment, illustrated in FIG. 5, differs from the previous one in that the winding, after hot rolling 1, takes place at a temperature t ₂ for which the microstructure is composed of ferrite / perlite or ferrite / perlite / bainite.

As an alternative to the embodiments of Figures 4 and

5, it is possible, as in the second embodiment and as indicated by dashed lines 3 ′ in these FIGS. 4 and 5, producing the tempering 3 at a temperature below the temperature Ai.

The sixth embodiment, illustrated in FIG. 6, differs from the previous one in that, on the one hand, the winding of the sheet, after hot rolling, takes place at a temperature higher than the temperature Ai and below the temperature A ^ + 50 ° C, this to keep a fraction of austenite unprocessed and that, on the other hand, this winding is followed by a maintenance substantially at this temperature under an isolation bell for at least 4 hours to enrich the unprocessed austenite with gamma elements, such as manganese and / or boron. The previous embodiments make it possible to obtain a hot rolled product (LAC) which can have several types of microstructures, the nature of which is linked to the chemical composition of the austenite formed and to the final cooling rate (TRC diagram). It is therefore a ferrite matrix with islands of at least one of the following phases: residual austenite, bainite and / or martensite. Advantageously, the hot-rolled steel can still undergo, as already mentioned above, after pickling either by an acid bath or by annealing in a reducing atmosphere, a step of electrochemical galvanization and / or metallization, so to obtain a sheet of galvanized or electrogalvanized steel, for example.

The embodiments of the method according to the invention illustrated diagrammatically by the graphs of FIGS. 7 to 15 relate to a coil which has undergone cooling substantially to ambient temperature, optionally followed by cold rolling with, for example , a reduction rate between 40% and 90%.

Thus, the seventh embodiment according to FIG. 7 is distinguished from that of FIG. 1 by the fact that a cooling 5 called "slow" or accelerated by quenching with possibly a conventional cold rolling 6, is interposed between the tempering 3 and heat treatment 4, which in this embodiment is formed by continuous intercritical annealing.

During this cold rolling 6, a reduction in the thickness of the sheet is generally carried out between 40 and 90% and a hardened microstructure, which recrystallizes during the subsequent annealing 4. If such cold rolling is provided, this is preceded by a pickling which can be done before or after the income operation 3.

It should also be noted that the sheet subjected to cold rolling 5 is, before annealing 4, essentially made up of ferrite and cementite rich in carburogenic elements, such as Mn, Cr, Mo and / or B. The embodiment according to FIG. 8 is a variant of the previous one, which is distinguished by the fact that the heat treatment 4 is formed by a discontinuous intercritical annealing, also called "bell intercritical annealing". The embodiments according to FIGS. 9 and 10 are distinguished from that of FIG. 8 by the fact that the income 3 is dissociated into a partial income 3a at a temperature below the temperature A1 and an income 3b between the temperature Ai and the temperature Ai + 50 ° C. In the embodiment according to FIG. 9, the heat treatment 4 is a continuous intercritical annealing, while in the embodiment according to FIG. 10, it is an intercritical annealing by bell annealing.

The two embodiments according to FIGS. 11 and 12 are distinguished from the previous embodiments by the fact that both the tempering 3 and the heat treatment 4 take place at a temperature above the temperature Ai and that, moreover , these two operations 3 and 4 are separated by cooling 5 and cold rolling 6.

In the embodiment according to FIG. 11, it is a continuous intercritical annealing 3, and, in that of FIG. 12, it is an intercritical annealing by annealing bell 3.

In these two embodiments, the austenite formed is enriched with so-called "gamma" elements, such as Mn, B and / or C, during tempering 3.

In the embodiment illustrated in FIG. 13, the hot rolling 1 is followed by a slight cooling 2 and a winding below the temperature Ai and above the temperature t ₂ of the start of the bainite formation to obtain a ferrite / perlite microstructure. This temperature of the coil is then maintained by thermal insulation in a bell for at least 4 hours, as in the embodiment illustrated in FIG. 6. The embodiment according to FIG. 14 is comparable to that of FIG. 4 or to that of FIG. 5, except that cooling 5, known as "slow" or accelerated by quenching, possibly with cold rolling 6 , is provided between tempering 3 and heat treatment 4. Heat treatment 4 can be carried out by continuous intercritical annealing or by bell intercritical annealing.

The embodiment according to FIG. 15 is distinguished from that of FIG. 13 by the fact that the hot rolling 1 is immediately followed by maintaining in an isolation bell at a temperature between the temperature Ai and the temperature Ai + 50 ° C to keep unprocessed austenite in the microstructure.

In the embodiments illustrated by the various figures, when the heat treatment 4 is formed by continuous intercritical annealing, this generally takes place at a temperature between the temperature Ai and 900 ° C for 1 second to 5 minutes, combined optionally either with metallization by quenching, or followed by metallization by electrochemical deposition. If the heat treatment 4 is formed by a bell-shaped intercritical annealing, this preferably takes place at a temperature lying between the temperature Ai and 50 ° C. above this temperature.

It has been found that the steel obtained according to the process described above, more particularly the particular embodiments illustrated by FIGS. 1 to 15, have in particular the following properties: - better surface quality and ability to galvanizing compared to multi-phase steels containing Si,

- a good relationship between the elastic limit and the breaking load,

- good forming properties, high uniform elongation,

an increase in the elastic limit and the breaking load by the effect of the secondary precipitation of the elements of micro-alloys during tempering compared to conventional multi-phase steels, - a high work hardening coefficient during a deformation,

- protection against necking, thanks to high n values and high uniform elongation,

- a weak or nonexistent level of vermiculure ("yield point elongation") allowing low "skinpass" lengthening,

- good behavior under mechanical fatigue by a combination of hard phases formed from bainite and martensite, and from soft phases, ferrite and residual austenite,

- a high energy absorption value during the deformation at high speed as a result of the multiplication mechanisms of dislocations.

In order to further illustrate the object of the present invention, below are given some concrete examples of chemical compositions of a multiphase steel according to the invention and parameters of the various stages of the preparation applied to this steel.

Example 1 This example was carried out according to the particular embodiment of the process illustrated by the graph in FIG. 7. Chemical composition:

0.16% C 1.60% Mn 0.40% Si

1) Reheating: temperature: 1280 ° C time: 150 min.

2) Coating for roughing: final temperature: 1100 ° C (1 ^st step)

3) Finishing rolling: final temperature: 850 ° C

4) Cooling table: cooling rate: 50 ° C / s 5) Winding temperature: below 400 ° C, natural air cooling

6) Stripping:

7) Annealing bell ^(Step 2) heating rate: 40 ° C / hour holding temperature: 640 ° C holding time: 40 hours Cooling rate: 15 ° C / h

8) Cold rolling: reduction rate: 60%

9) Continuous Annealing: ^(Step 3) heating rate: 15 ° C / s holding temperature: 740 ° C Holding time: 50 sec cooling rate: 40 ° C / s elongations of skinpass: 0.2%

10) Galvanization by passage through a bath of liquid zinc.

FIG. 16 schematically shows a cross section of the steel according to this example provided with a coating 8 of zinc obtained by galvanization. This steel comprises a ferrite matrix 9 in which are distributed substantially uniformly islands 10 of residual austenite of substantially equivalent dimensions.

Mechanical properties obtained during an industrial test on a sheet sample 1.5 mm thick taken from the center line of the sheet in the direction of rolling subjected to a tensile test according to ISO standard with test piece, base 80 mm, width 20 mm.

Yield strength Rp 0.2% (MPa) 370 Breaking load Rm (MPa) 580 Rp ratio 0.2% / Rm (%) 63.8 Bearing elongation (%) 1, 1 uniform elongation (%) 18 , 8 Total elongation (%) 31, 2 Work hardening coefficient n 0.262 Anisotropy coefficient r 1, 32

Example 2

This example was carried out according to the particular embodiment of the process illustrated by the graph in FIG. 3. Chemical composition: 0.10% C 0.20% Mn 1.00% Si 0.06% Ti 0.050% B

1) Reheating: temperature: 1300 ° C time: 200 min.

2) Coating for roughing: final temperature:

1100 ° C (1 ^st step)

3) Finishing rolling: final temperature: 860 ° C

4) Cooling table: cooling rate: 25 ° C / s

5) Winding temperature: below 300 ° C, natural air cooling

6) Stripping:

7) Bell annealing: heating speed 100 ° C / h holding temperature 740 ° C holding time 200 h cooling speed 25 ° C / h

Figure 17 shows schematically the structure of the steel obtained according to this example. This steel comprises a ferrite matrix 9 in which are distributed substantially uniformly the bainite islands 10 of substantially equivalent dimensions.

Example 3 This example was carried out according to the particular embodiment of the process illustrated by the graph in Figure 7 Chemical composition: 0.80% C 3.00% Mn 0.2% Si 1) Reheating: temperature: 1150 ° C time: 135 min.

2) Coating for roughing: final temperature: 1000 ° C (1 ^st step)

3) Finishing rolling: final temperature: 750 ° C

4) Cooling table: cooling rate: 40 ° C / s

5) Winding temperature: below 200 ° C, natural air cooling

6) Stripping

7) Annealing bell ^(Step 2) Heating speed 60 ° C / hour holding temperature 300 ° C holding time 10 h cooling rate 50 ° C / h

8) Cold rolling: reduction rate 40%

9) Continuous annealing: heating rate 10 ° C / s holding temperature 730 ° C holding time 240 sec cooling rate: 100 ° C skinpass elongations:> 0.2%

10) Electro-zinc plating

FIG. 18 schematically shows the microstructure of the steel obtained according to this example provided with a zinc coating 8 formed by electrodeposition. This steel has a ferrite matrix 9 in which are distributed substantially uniformly martensite islands 10 of very small dimensions.

Example 4 This example was carried out according to the particular embodiment of the method illustrated by the graph in FIG. 7

Chemical composition

0.170% C

1.800% Mn 0.020% Ti

0.080% V

0.003% B

1) Reheating at 1280 ° C, for 150 min

2) Coarse rolling: final temperature of 1100 ° C 3) Finishing rolling: final temperature of 910 ° C up to a thickness of 2.0 mm

4) Cooling table: cooling rate of 50 ° C / s

5) Winding temperature: temperature of 350 ° C, cooling of the coil in air to room temperature 6) Classic pickling, i.e. with the coil unwound in hot HCI 7) Basic annealing: rise of 40 ° C / hour holding temperature of 520 ° C holding time of 40 h cooling rate of 15 ° C / hour to room temperature 8) Cold rolling with a reduction rate of 60%

9) Continuous annealing: rise of 15 ° C / s holding temperature of 820 ° C holding time 46 s cooling speed of 30 ° C / s skinpass level of 0.6%

10) Metallization: conventional galvanization combined with continuous annealing The mechanical properties obtained during an industrial test on a sheet sample of a thickness of 1.5 mm taken from the axis of the sheet in the direction of rolling subjected to traction of 20/80 (ISO Standard) are as follows: Yield strength RpO. 2% (MPa): 471

Breaking load Rm (MPa): 649 Ratio RpO. 2% / Rm (%) 72.6

Bearing elongation (%) 1, 0

Uniform elongation (%) 14, 1

Total elongation (%) 28.3

Work hardening coefficient n 0.221 Figure 19 shows schematically the structure of the steel obtained according to this example provided with a zinc coating 8 formed by galvanization. This steel has a ferrite matrix 9 in which are distributed substantially uniformly islands 10 of residual austenite and precipitates 11 of vanadium carbides. Example 5

This example was carried out according to the particular embodiment of the process illustrated by the graph in FIG. 13. Chemical composition 0.800% C 3.000% Mn 1) Reheating at 1100 ° C for 200 min

2) Coarse rolling: final temperature of 1000 ° C

3) Finishing rolling: final temperature of 740 ° C up to a thickness of 1.5 mm 5 4) Cooling table: cooling rate of 10 ° C / s

5) Winding temperature: temperature of 620 ° C, maintained by isolation bell

6) Insulation bell: holding temperature of 620 ° C holding time of 168 h o cooling speed of

25 ° C / hour

7) Cold rolling with a reduction rate of 50%

8) Continuous annealing: rise of 10 ° C / s holding temperature of 850 ° C 5 holding time 40 s cooling rate of 25 ° C / s without skinpass

9) Metallization: electro-zinc coating.

FIG. 20 schematically shows the structure of the steel 0 obtained according to this example provided with a zinc coating 8 formed by electrodeposition. This steel comprises a ferrite matrix 9 in which are distributed substantially uniformly islands of martensite

10.

Example 6 5 This example was carried out according to the particular embodiment of the process illustrated by the graph in FIG. 14. Chemical composition 0.100% C 0.200% Mn 0 0.400% Si 0.040% Ti

0.050% Nb

0.200% Cr

0.200% Mo 1) Reheating at 1300 ° C for 135 min

2) Coarse rolling: final temperature of 1100 ° C

3) Finishing rolling: final temperature of 890 ° C up to a thickness of 3.0 mm

4) Cooling table: cooling rate of 50 ° C / s 5) Coiling temperature: temperature of 400 ° C, direct application of base annealing

6) Base annealing: rise of 60 ° C / h holding temperature of 580 ° C holding time of 20 h cooling rate of

50 ° C / hour

7) Classic stripping

8) Cold rolling with a reduction rate of 70%

9) Continuous annealing: rise of 10 ° C / s holding temperature of 740 ° C holding time 240 s cooling speed of 70 ° C / s skinpass level of 0.6% Figure 21 shows schematically the structure of the steel obtained according to this example. This steel comprises a ferrite matrix 8 in which are distributed substantially uniformly islands 10 of martensite and bainite as well as precipitates 11 of titanium carbides and niobium carbides.

Example 7 This example was carried out according to the particular embodiment of the method illustrated by the graph in FIG. 6. Chemical composition 0.150% C 2,000% Mn 0.030% B 1) Reheating at 1280 ° C for 150 min

2) Coarse rolling: final temperature of 1100 ° C

3) Finishing rolling: final temperature of 840 ° C up to a thickness of 6.0 mm

4) Cooling table: cooling rate of 15 ° C / s 5) Winding temperature: temperature of 740 ° C, holding by isolation bell 6) Isolation bell: holding temperature of 740 ° C holding time 40 h accelerated cooling rate of 5 ° C / s

FIG. 22 schematically shows the structure of the steel obtained according to this example provided with a film of scale 8. This steel has a ferrite matrix 9 in which are distributed substantially uniformly islands 10 of martensite and residual austenite.

Example 8 This example was carried out according to the particular embodiment of the process illustrated by the graph in FIG. 5. Chemical composition 0.400% C

0.600% Mn 0.020% Ti 0.002% B 0.400% Al 0.400% Si 1) Reheating at 1250 ° C for 200 min

2) Coarse rolling: final temperature of 1080 ° C

3) Finishing rolling: final temperature of 800 ° C up to a thickness of 2.5 mm 4) Cooling table: cooling rate of 20 ° C / s

5) Coiling temperature: temperature of 600 ° C, direct application of base annealing

6) Base annealing: rise of 30 ° C / h holding temperature of 750 ° C holding time of 100 h natural air cooling

7) Classic pickling.

FIG. 23 shows diagrammatically the structure of the steel obtained according to this example comprising a ferrite matrix 9 in which are distributed substantially uniformly islands of bainite.

It is understood that the invention is not limited to the embodiment of the process for the specific preparation of a steel, but, on the other hand, extends to any steel having substantially the same structure and morphology as steel multiphase obtained by the specific process described above and illustrated by the accompanying figures.

Claims

1. Flat product, such as sheet metal, made of multiphase steel whose chemical composition contains carbon and manganese, characterized in that this steel has a structure capable of being obtained by a process comprising the following steps:

- hot rolling, at a temperature at which the austenitic phase is stable, of a steel containing from 0.05 to 0.8% of C, 0.2 to 3.0% of Mn and Si <1%,

- maintaining or returning to a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (Ai) for at least 4 hours, possibly followed by cooling to room temperature, so as to form a phase enriched in carburigenic and / or gamma elements, such as carbon and manganese in a ferrite matrix, - a heat treatment at a temperature higher than the above eutectoid temperature (Ai) and lower than the formation temperature of austenite (A3) so as to form islands of austenite and / or to enrich with gamma elements such as Mn of the austenite already formed, and - subsequent cooling to room temperature in a way such as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite.

2. Product according to claim 1, characterized in that the aforementioned method comprises a pickling step after hot rolling.

3. Product according to claim 1, characterized in that the aforementioned method comprises a pickling step after the step of maintaining or tempering at a temperature between 300 ° C and a temperature of 50 ° C above the temperature eutectoid (Ai) or after the above heat treatment.

4. Product according to any one of claims 1 to 3, characterized in that the method comprises a step of cooling after hot rolling at a speed and to a temperature below the temperature Ai which are such to obtain a microstructure containing either bainite and / or martensite, or perlite with possibly ferrite.

5. Product according to any one of claims 1 to 3, characterized in that, when the aforementioned hot rolling makes it possible to obtain a rollable sheet, this rolling is followed by a winding of the sheet obtained at a temperature between the temperature eutectoid (Ai) and a temperature of 50 ° C above this eutectoid temperature, so as to keep unprocessed austenite in the microstructure obtained.

6. Product according to any one of claims 1 to 3, characterized in that the steel subjected to the aforementioned hot rolling also comprises at least one of the following elements: B, Ti, Nb, Zr, V, Al, N, S, P, Cr, Ni, Cu, Mo in the following concentrations: B <0.100%; Ti, Nb, Zr or V each <0.200%, Al <0.400%; N <0.100%; P <0.100%; Cr, Ni or Cu each <2,000%; Mo <0.500%.

7. Product according to any one of claims 1 to

6, characterized in that the steel subjected to the aforementioned hot rolling contains at most 0.5% of Si, more particularly less than 0.2% of Si, and is preferably substantially free of Si.

8. Product according to either of Claims 6 or 7, characterized in that the following elements: Al, N, B respectively have the following concentrations: Al <0.100%; N <0.05%; B <0.060%; Ti, Nb, Zr or V each <0.100%; Cr <0.500%.

9. Product according to any one of claims 1 to 8, characterized in that it has a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite, the content of this phase in the ferrite matrix being less than 40% by volume.

10. Product according to any one of claims 1 and 9, characterized in that the aforementioned process comprises a step of cooling down to ambient temperature after the aforementioned maintenance or tempering step, this last step then being optionally followed a cold rolling step before the aforementioned heat treatment step.

11. A method for manufacturing a flat product, such as a steel sheet, according to any one of claims 1 to 10, characterized in that it comprises the following steps:

- hot rolling, at a temperature at which the austenitic phase is stable, of a steel containing from 0.05 to 0.8% of C, 0.2 to 3.0% of Mn and Si <1%, - tempering or maintaining at a temperature between 300 ° C and a temperature of 50 ° C above the eutectoid temperature (Ai) for at least 4 hours, possibly followed by cooling to room temperature, so as to form a phase enriched in gamma and / or carburigenic elements, such as C and Mn in a ferrite matrix,

- a heat treatment higher than the above eutectoid temperature (A ^ and lower than the austenite formation temperature (A3) so as to form islands of austenite and / or to enrich with gamma elements, such as Mn of the austenite already formed, and - subsequent cooling down to room temperature in such a way as to obtain a final product having a ferrite matrix containing islands of at least one of the following phases: residual austenite, bainite and martensite .

12. Method according to claim 11, characterized in that the above-mentioned tempering is carried out at a temperature below the eutectoid temperature (Ai) so as to form cementite and to diffuse carburogenic elements, such as Mn in this cementite.

13. Process according to claim 11, characterized in that the aforementioned tempering is carried out at a temperature higher than the eutectoid temperature (Ai) so as to form austenite and to diffuse gamma elements in this austenite.

14. Method according to claims 11 or 12, characterized in that the aforementioned tempering is followed, before the aforementioned heat treatment, by cooling to room temperature in such a way as to slow down the enrichment of the phase already enriched with gamagenic and / or carburogenic elements.

15. Process according to claim 13 or 14, characterized in that the heat treatment is formed by continuous annealing at a temperature of at most 900 ° C for at most 5 minutes, so as to transform the enriched phase into gamma-elements and / or carburigens in austenite whose chemical composition corresponds substantially to that of the phase enriched with gamma-elements and / or carburigens, this annealing is then followed by rapid cooling, for example of the order of 10 ° C. to 300 ° C per second.

16. The method of claim 11, characterized in that the aforementioned income is directly followed, without intermediate cooling, the aforementioned heat treatment, formed by a bell annealing for at least 2 hours, in such a way as to transform cementite in austenite, this heat treatment then being followed by cooling, for example of the order of 10 ° C to 40 ° C per hour or accelerated cooling by quenching, for example of the order of 20 ° C per minute.

17. Process according to any one of claims 11 to 16, characterized in that it comprises a cooling step between the hot rolling and the above maintenance or tempering.

18. Process according to claim 17, characterized in that the above-mentioned cooling is followed by pickling and possibly cold rolling before the above-mentioned heat treatment.

19. Method according to any one of claims 11 to 18, characterized in that the heat treatment is formed by bell annealing and is followed by slow cooling; for example of the order of 10 ° to 40 ° C per hour or of accelerated cooling by quenching, for example of the order of 20 ° C per minute.

20. The method of claim 18 or 19, characterized in that the cold rolling is followed by annealing at a temperature between the eutectoid temperature (Ai) and the austenite formation temperature (A ₃ ), this annealing then being followed by rapid cooling, for example of the order of 10 ° C to 300 ° C per second.

21. Process according to any one of claims 11 to 20, characterized in that the aforementioned income is produced in two successive stages, a first stage at a temperature between 300 ° C and the eutectoid temperature (Ai) and a second stage at a temperature between at the eutectoid temperature (Ai) and a temperature of 50 ° C above this temperature Ai.

22. Process according to any one of Claims 11 to 20, characterized in that the aforementioned tempering or maintenance and the aforementioned heat treatment are carried out in a single operation at a temperature Ai and 50 ° C above this temperature Ai.

23. Method according to any one of claims 11 to 16 and 18 to 22, characterized in that the hot rolling is directly followed by tempering either at a temperature above the temperature Ai and below the temperature Ai + 50 ° C either at a temperature below the temperature Ai.

24. Method according to any one of claims 11 to 22, characterized in that it comprises, between the hot rolling and the holding or tempering above, cooling and winding either at a temperature at which the microstructure is composed by bainite and / or martensite, or at a temperature below the temperature Ai at which the microstructure is composed of ferrite / perlite or ferrite / perlite / bainite.

25. Process according to any one of claims 11 to 24, characterized in that the heat treatment is combined or followed by a step of forming a metallic coating, such as galvanization or electrochemical metallization.